A fuel supply device, which pressurizes fuel stored in the fuel tank and discharges the pressurized fuel to the internal combustion engine through use of a fuel pump, is known from, for example, the patent literature 1. The device disclosed in the patent literature 1 has a swirling wall structure that swirls a fuel flow.
Specifically, the swirling wall structure of the device disclosed in the patent literature 1 is placed along a path, which extends from the fuel pump to the internal combustion engine, to swirl the fuel flow about an axis that extends in a vertical direction. In this way, air bubbles, which are contained in the fuel and have a small specific gravity, are concentrated in a center part of the swirl flow, so that a cluster of air bubbles is formed in the center part of the swirl flow in a manner that increases a buoyant force exerted to the air bubbles. Thus, the cluster of air bubbles is expelled from the path, which extends from the fuel pump to the internal combustion engine, through a vent hole, which extends through an upper wall of the swirling wall structure. Thereby, in the internal combustion engine, it is possible to limit deterioration of a performance, which would be caused by the intake of the fuel containing the air bubbles.
However, in the swirling wall structure of the device disclosed in the patent literature 1, the presence of the vent hole in the path, which extends between the fuel pump and the internal combustion engine, causes that a portion of the fuel to be supplied to the internal combustion engine is escaped through the vent hole, so that a sully loss of the fuel is induced. The fuel supply loss of this kind causes wasting of the drive energy of the fuel pump. Therefore, there is a need for improvement in view of the energy saving. When the leakage of the fuel is reduced by reducing a diameter of the vent hole, the air bubbles cannot be effectively expelled through the vent hole having the reduced diameter. Thereby, the air bubbles may remain in the fuel to possibly cause deterioration of the performance of the internal combustion engine.